review2.md

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Review-2

Lecture Date : 08/30/16

Practice

1. Iterators

1a. The following code has a bug. The countdown stops at 10 (no 9,8,7...). Why?

def countdown(n):
   while n >= 0:
     return  n
     n -= 1

print("We are go for launch")
for x in countdown(10):
   print(x)
print("lift off!")

1b. Modify the following code such that the final out list only contains numbers over 20

def items(x, depth=-1):
  if isinstance(x,(list,tuple)):
    for y in x:
      for z in items(y, depth+1):
        yield z
  else:
    yield x

out = []
for x in items(  [10,[ 20,30],
                        40,
                        [   (  50,60,70),
                            [  80,90,100],110]]):
   out += [x]
print out

1c. Repeat the above, this time using list comprehensions.

1d. Using list comprehensions, write a function that returns only non-whitespace in a string. Hint:

import string
string.whitespace # <== contains all whitespace chars

1e. Using list comprehensions and the following code, return all lines in a multi-line strings that are (a) non-blanks and (b) longer than 20 characters in length. Hints: not str returns True for non empty strings.

def lines(string):
  tmp=''
  for ch in string: 
    if ch == "\n":
      yield tmp
      tmp = ''
    else:
      tmp += ch 
  if tmp:
  yield tmp

1f. Whats the logical error in the code? I need a list of 20 random numbers

import random
x = []
for _ in range(20):
  random.seed(1)
  x.append(random.random())
print(x)

Correct it by moving 1 line.

1g. Write a small snippet for a random odd integer generator between 1 and 1000. (Hint: yield)

2. Dunders

2a. What are the dunders in the following code? For each one, use them in a code snippet.

class o:
  def __init__(i,**d)    : i.__dict__.update(d)
  def __setitem__(i,k,v) : i.__dict__[k] = v
  def __getitem__(i,k)   : return i.__dict__[k]
  def __repr__(i)        : return 'o'+str(i.__dict__)

2b. In the above, what is the magic dict variable?

2c. What would happen if the last line in the following __iadd__ method was deleted?

r = random.random
rseed = random.seed

class Some:
  def __init__(i, max=8): 
    i.n, i.any, i.max = 0,[],max
  def __iadd__(i,x):
    i.n += 1
    now = len(i.any)
    if now < i.max:    
      i.any += [x]
    elif r() <= now/i.n:
      i.any[ int(r() * now) ]= x 
    return i

2d. In English, explain what the above Some class does. Use it in a loop to keep Some numbers in the series 0,1,2,...999.

3. 8 Queens

You've seen the knightstour. Can you code up the 8 queens problem in python? (https://en.wikipedia.org/wiki/Eight_queens_puzzle)

4. FSM

What is a finite state machine? Can you describe an fsm in python for the following problem. You're in a bar.

1. You start of sober.
2. If you are sober you take a drink
3. If you are drunk and take a drink then there is a 80% chance you get drunk and there is a 20% chance you pass out.
4. If you are drunk and do not take a drink, there is a 50% chance you get sober.
5. If you pass out you obviously stop drinking